The road to cleaner energy

Passage of the Clean Air Act in 1970 began a slow cleanup of sooty pollution from the smokestacks of coal-fired power plants, which generate about half of the nation's electricity. Plants also are releasing fewer of the chemicals that cause smog and acid rain.

But those efforts created other problems, including more toxic byproducts like the muck that recently spilled in Tennessee and Alabama.

A push is growing nationally to figure out ways to burn coal while curbing CO2 emissions. Last month, Illinois took a step in that direction when then-Gov. Rod Blagojevich signed a law that proposes a new Downstate power plant that would capture half of its carbon dioxide emissions.

If built, the plant would be the first major test of such technology.

Coal legislation: 1970s

Congress enacted the Clean Air Act in 1970, which mandated that states and the federal government regulate emissions from mobile sources (cars, trucks) and from industrial sources, including coal-fired power plants.

Shortly thereafter, the Environmental Protection Agency was created to enforce the new law.

Production, emissions: U.S. coal production in billions of short tons, Carbon dioxide emissions in billions of metric tons

- See microfilm for complete graphic

Technological solutions to dirty coal: Removing ash

The first efforts to curb pollution from coal-fired power plants involved removing relatively large pieces of ash from coal exhaust. The buildup of unwanted ash created another problem: toxic sludge.

ELECTROSTATIC PRECIPITATOR

Uses electrically charged plates to filter coal exhaust

1. Coal plant exhaust enters unit

2. Ash particles cling to electrodecharged collector plates.

BAGHOUSE

A series of cloth bags filter particulate matter (large pieces of ash) from the rest of the exhaust.

1. Coal plant exhaust enters unit.

2. Dust particles enter up and are trapped in the filtered bags. (cutaway)

A motorized shaker mechanism empties the bags when they are full.

3. Cleaned gas is expelled.

Coal legislation: 1980s

Emissions laws were essentially unchanged under the Reagan administration. But in 1986, the Department of Energy initiated the Clean Coal Technology Program in response to the growing problem of acid rain.

The program resulted in the developement of 35 experimental projects to reduce sulfur dioxide and nitrogen oxide emissions.

Production, emissions: U.S. coal production in billions of short tons, Carbon dioxide emissions in billions of metric tons

- See microfilm for complete graphic

Technological solutions to dirty coal: New way of burning coal

One of the major technological developments in the 1980s was a type of coal burner that reduced emissions of both nitrogen oxide and sulfur dioxide. But carbon emissions remained.

FLUIDIZED BED BOILER

A system designed to mix coal with limestone in suspended air, reducing sulfur content.

1. Coal and limestone are fed into the boiler. The limestone absorbs the sulfur in air creating calcium sulfate and reducing sulfur emissions.

2. Jets force the coal upward, suspending it in air inside a chamber where it is ignited by burners. The burners are cooler than a regular boiler, preventing the nitrogen from breaking up and creating nitrogen oxides.

3. Clean exhaust is expelled to heat water into steam.

Decrease in emissions

Sulfur dioxide: 2006: 9,277

Nitrogen oxides: 2006: 3,199

In millions of short tons

Coal legislation: 1990s

In 1990, Congress passed several amendments to the Clean Air Act, expanding the federal government's authority to enforce the regulations and creating new programs to control acid deposition and nearly 200 toxic pollutants.

Production, emissions: U.S. coal production in billions of short tons, Carbon dioxide emissions in billions of metric tons

- See microfilm for complete graphic

Technological solutions to dirty coal: Targeting sulfur

In the 1990s, shifts in resources and cleaning technologies by coal companies underscored efforts to further reduce sulfur emissions.

SCRUBBER

Washes dirty sulfur out of gases.

1. Combustion gases containing sulfur dioxide enter.

2. A scrubbing slurry mix of limestone and water is sprayed onto gases, trapping the sulfur and creating a sludge that collects on the bottom.

3. Gases are washed, and mist is eliminated.

4. Clean gas reheated and eliminated.

A strategic shift

As part of the effort to reduce sulfur emissions, many companies began using coal from western regions, where the coal contains less sulfur.

U.S. COAL PRODUCTION, In millions of tons

2007: 668

2007: 477

U.S. COAL DEPOSITS

West of Mississippi River, East of Mississippi River

ACID RAIN TRADING PROGRAM

Companies were allowed to emit a specific amount of sulfur dioxide and could purchase the rights from other companies to emit above that level.